We created a series of transgenic mice which either overexpress methionine sulfoxide reductase A in specific cellular locations or which lack the enzyme. We also established a mouse lacking methionine sulfoxide reductase B1 and successfully crossed it to create a double knockout mouse lacking both the A and B1 isozymes. The role of these reductases in resistance to oxidative stress during ischemic-reperfusion was studied with the Langendorff model. We expected that the transgenic mouse overexpressing methionine sulfoxide reductase A in mitochondria would be protected against injury while one or more of the knockout animals would experience more severe injury. This is not what we found. Rather, the knockout animals experienced the same degree of injury as the wild-type mice. Further, only the transgene expressing cytosolically targeted methionine sulfoxide reductase A was protected from ischemia-reperfusion induced cardiac injury. Importantly, the enzyme must be myristoylated to provide protection. A transgenic mouse overexpressing a non-myristolated methionine sulfoxide reductase A was not protected from injury. We are also completing an investigation of the hypothesis that methionine sulfoxide reductase is important in nutrition, functioning to salvage the metabolically "expensive" amino acid methionine by reducing its sulfoxide. Weanling animals with different genotypes were raised on diets varying in methionine content, and their growth and biochemical parameters were being measured. This large body of data is now being analyzed. While methionine sulfoxide reductase A is well-known to reduce methionine sulfoxide to methionine, we have now demonstrated that it is capable of catalyzing the reverse reaction: methionine sulfoxide reductase A is also a methionine oxidase. The enzyme stereospecifically oxidizes free methionine, methionine in peptides, and methionine in proteins. Because the reaction is stereospecific, the enzyme can also fully reverse the reaction. This finding establishes a mechanistic basis for methionine sulfoxide reductase A to function in cellular regulation and signaling.